Hann window incorrectly defined

[hugke729]

The Hann window in arduinoFFT.cpp is defined as 0.54 * (1.0 - cos(twoPi * ratio)). The 0.54 should be 0.5 (e.g., Hann function on Wikipedia). Looks like the incorrect 0.54 perhaps originates from the Hamming window, where 0.54 is the correct value

Not sure if this alters the respective value in the window compensation table in #40.

/cc @pvutukur